Reverse tensioned mooring system

ABSTRACT

In a method of mooring floating structures a chain receiving device is secured to an anchor and a forerunner chain is extended through the chain receiving device, after which the anchor is secured in engagement with the sea floor. A mooring line is extended between a floating structure and the forerunner chain. A tensioning device on an anchor handling vessel is employed to apply mooring tension to the forerunner chain thereby applying mooring tension to the mooring line secured between the forerunner chain and the floating structure. Mooring tension is maintained by the chain receiving device on the anchor.

CLAIM OF PRIORITY

This application claims priority of provisional application Ser. No. 60/701,369 filed Jul. 21, 2005, currently pending, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to systems for mooring floating structures, and more particularly to a mooring system which eliminates the requirement for providing tensioning mechanisms on floating structures to be moored, and in lieu thereof utilizes tensioning mechanisms mounted on one or more anchor handling vessels to apply mooring tension.

BACKGROUND AND SUMMARY OF THE INVENTION

Traditionally, floating structures such as drilling rigs, production rigs, CALM buoys, SPARs, FPSOs, FSOs, etc. have been moored to an anchor system that is put under operating tension by tensioning equipment on the floating structure. For large surface structures the mooring tensioning equipment usually contains winches, fairleads, chain lockers, presets, and excess wire reels. When the floating structure is brought onto location, the anchors are run out, or alternatively, the preset mooring lines are connected to the anchors (the so called preset anchor system).

Once the floating structure is connected to the anchors, by either method, the mooring legs are tensioned to the design specifications and the structure is secured at the desired location. Thus, tensioning is done at the structure for large floating structures. For smaller structures, such as CALM buoys, the tensioning is done by temporary winches or cranes. The concept is the same; the tensioning is done at the floating structure.

For example, in the case of suction anchors, the mooring padeye is commonly located two-thirds of the way down the side of the anchor. The suction anchor is preinstalled and the anchor forerunner chain is attached to the mooring padeye. The end of the forerunner chain is run out and extends to a subsea connector which is typically positioned on the sea floor. When the floating structure is brought on location, the mooring line is connected to the subsea connector and the mooring leg is tensioned by tensioning equipment on the floating structure. Plate anchors, driven piles, and other vertically loaded anchors have the same arrangement and means of tensioning.

The mooring system of the present invention differs in that tensioning is done at the anchors with the aid of tensioning equipment on an anchor handling vessel. There is no need for any tensioning equipment on the floating structure. The mooring line is connected to the floating structure by means of a simple, fixed padeye or similar appurtenance. No winches, fairleads, etc. are needed on the floating structure.

For each anchor type the modifications necessary in order to implement the present invention are the same. The padeye is replaced with a turning shoe or chain whelp. In addition to the shoe or whelp, a stopper mechanism such that the chain can pass in one direction only (away from the floating structure) is incorporated. If the anchor has a portion of its structure above the mudline the stopper may be located at the top of the anchor. For completely embedded anchors the stopper may be incorporated into the turning shoe or whelp. The stopper mechanism has a release line or mechanism such that the mooring line tension can be released, if necessary. The forerunner chain is run through the turning shoe or whelp and back to the surface, either on top of the pile or suction anchor, or on the sea floor for plate anchors or vertically loaded anchors. The subsea connector arrangement is the same as with conventionally tensioned mooring systems.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be had by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings, wherein:

FIG. 1 is an illustration of a prior art mooring system;

FIG. 2 is an illustration of an initial step in a method of mooring floating structures comprising a first embodiment of the invention;

FIG. 3A is an illustration of a somewhat later step in the method of mooring floating structures comprising the first embodiment of the invention;

FIG. 3B is an illustration of a still later step in the method of mooring floating structures comprising the first embodiment of the invention;

FIG. 3C is an illustration of a still later step in the method of mooring floating structures comprising the first embodiment of the invention wherein a tensioning device on an anchor handling vessel is connected to a mooring line extending to a floating structure;

FIG. 3D is an illustration of a still later step in the method of mooring floating structures comprising the first embodiment of the invention wherein the tensioning device of the anchor handling vessel is utilized to apply mooring tension to the mooring line extending to the floating structure;

FIG. 4A is an illustration of an initial step in a method of mooring floating structures comprising a second embodiment of the invention; and

FIG. 4B is an illustration of a later step in the method of mooring floating structures comprising the second embodiment of the invention.

DETAILED DESCRIPTION

Referring to the drawings, and particularly to FIG. 1 there is shown a conventional floating structure 10. Although a particular type of floating structure is illustrated in FIG. 1, those skilled in the art will understand that the floating structure 10 may comprise a drilling rig, a production rig, a CALM buoy, a SPAR, an FPSO, an FSO, etc. As is also well known, larger floating structure are typically provided with a plurality of permanently installed tensioning devices 12 such as winches or the like. In the case of smaller floating structures, such as CALM buoys, one or more tensioning mechanisms may be temporarily installed thereon for use during the mooring process.

Regardless of the type of tensioning devices that are installed on the floating platform 10 and regardless of whether the tensioning devices are temporarily or permanently installed, the tensioning devices are utilized to apply mooring tension to a plurality of mooring lines 14 which extend from the floating structure 10 to a plurality of anchors 16 each individual to one of the mooring lines 14. Those skilled in the art will appreciate the fact that the anchor 16 illustrated in FIG. 1 is a suction pile of the type shown and described in U.S. Pat. No. 6,009,825 the disclosure of which is incorporated herein by reference. However, the mooring system of FIG. 1 can also be utilized in conjunction with other anchor types such as plate anchors, driven piles, vertically loaded anchors, etc., all of which are well known in the art.

The method of mooring floating structures of FIG. 1 begins with the installation of a plurality of anchors at spaced apart locations around a mooring location. When the anchor is installed, a forerunner chain connected thereto is extended along the sea floor in the general direction of the mooring location. After all of the necessary anchors are installed, the floating structure 10 is moved to the mooring location and the mooring lines 16 thereof are connected to the forerunner chains extending from the previously installed anchors 16. At this point the tensioning mechanisms 12 mounted on the floating structure 10 are employed to apply mooring tension to the mooring lines 14. After the necessary mooring line tension has been achieved, the tensioning mechanisms 12 mounted on the floating structure 10 serve no further purpose unless and until the floating structure 10 is repositioned or relocated.

FIGS. 2, 3A, 3B, 3C, and 3D illustrate a method of mooring floating structures comprising a first embodiment of the present invention. Referring particularly to FIG. 2, an otherwise conventional suction anchor 20 is lowered into engagement with the sea floor by a line 22 extending from an anchor handling vessel. The suction anchor 20 is provided with a chain receiving device 24 which may comprise a turning shoe, a chain whelp, etc. A chain 26 extends around the device 24. One end of the chain is secured to the suction anchor 20 by a chain stopper 28. The opposite end of the chain includes a subsea connector 30 which is initially connected to the anchor handling vessel by a line 32.

As the line 22 is payed out from the anchor handling vessel, the suction anchor 22 initially penetrates the uppermost layers of the sea floor under its own weight. Thereafter a pumpskid 40 is engaged with the suction anchor 20. The pumpskid 40 is preferably of the type illustrated and described in U.S. Pat. No. 5,927,904 the disclosure of which is incorporated herein by reference. The function of the pumpskid 40 is to pump water out of the interior of the suction anchor 20 whereby the suction anchor 20 is driven downwardly into the sea floor under the action of the pressure that is imposed upon the suction anchor 20 by the sea. Operation of the pumpskid 40 and downward movement of the suction anchor 20 into the sea floor continue until only the uppermost portion of the suction anchor 20 remains exposed above the upper surface of the sea floor.

Referring to FIG. 3A, the suction anchor 20 is shown fully installed in the sea floor. The forerunner chain 26 extends upwardly from the device 24, and then along the surface of the sea floor to the subsea connector 30. An anchor handling vessel 42 connects a mooring line 44 to a floating structure 46 in the conventional manner. The floating structure 46 may comprise many of a wide variety of floating structures such as a drilling rig, a production rig, a CALM buoy, a SPAR, an FPSO, an FSO, etc. The floating structure 46 is identical to the floating structure 10 of FIG. 1 except that the floating structure 46 is not equipped with conventional mooring tensioning equipment.

Referring to FIGS. 3A and 3B, the mooring line 44 is initially secured to the anchor handling vessel 42 by a line 48. The mooring line 44 extends to a subsea connector 50 adapted for mating engagement with the subsea connector 30. A remotely operated vehicle (ROV) 52 operatively secured to the anchor handling vessel 42 is employed to connect the subsea connector 50 to the subsea connector 30, and to confirm that the connection has been properly made. After the subsea connectors 30 and 50 are joined one to another, the ROV 52 disengages the line 48 from the mooring line 44. After disengaging the line 48 from the mooring line 44, the ROV 52 connects the line 48 to the end of the chain 26 remote from the subsea connector 30 in the manner illustrated in FIG. 3C.

After the line 48 is connected to the chain 26, the anchor handling vessel 42 applies an upwardly directed force to the line 48 in the manner indicated by the arrow 54 in FIG. 3D. This causes the chain 26 to be pulled through the device 24 thereby applying mooring tension to the mooring line 44 and the floating structure 46. As will be appreciated by those skilled in the art, the chain stopper 28 (FIG. 2) prevents the chain 26 from moving in the reverse direction through the device 24. However, the chain stopper 28 is preferably constructed such that the chain 26 can be disengaged therefrom by the ROV 52 should it become necessary to release mooring tension from the line 44 and the floating structure 46 for any reason. The final step in the method of mooring floating structures according to the first embodiment of the invention comprises utilization of the ROV 52 to disengage the line 48 from the chain 26 whereupon the line 48 and the ROV 52 are withdrawn onboard the anchor handling vessel 42.

FIGS. 4A and 4B illustrate a method of mooring floating structures comprising a second embodiment of the invention. Many of the component parts utilized in the practice of the second embodiment of the invention are substantially identical in construction and function to component parts utilized in the practice of the first embodiment of the invention and described hereinabove in conjunction therewith. Such identical component parts are designated in FIGS. 4A and 4B with the same reference numerals utilized in the foregoing description of the first embodiment of the invention but are differentiated therefrom by a prime (′) designation.

Referring particularly to FIG. 4A, in the practice of the method of the second embodiment of the invention the suction pile 20′ does not, in and of itself, serve as an anchor. Rather, the suction pile 20′ serves as an installation device for a plate anchor 60. The construction of the plate anchor 60 and the utilization of the suction pile 20′ to effect installation thereof in the sea floor are fully described in U.S. Pat. No. 6,122,847 issued Sep. 26, 2000 the disclosure of which is incorporated herein by reference.

In accordance with the present invention the plate anchor 60 is provided with a chain 26′. During installation of the plate anchor 60, one end of the chain 26′ is secured to the suction pile 20 by a chain keeper 28A. The opposite end of the chain 26′ extends to a subsea connector 30′ which is in turn connected to an anchor handling vessel by a line 32′. The chain 26′ extends through a device 62 which serves both as a turning shoe or chain whelp and as a chain stopper.

FIG. 4B illustrates the plate anchor 60 installed in the sea floor. The plate anchor 60 may be provided with a recovery device 64. If used, the recovery the device 64 is deployed on the sea floor and is connected to the plate anchor 60 by lines 66. The purpose of the recovery device 64 is to allow the plate anchor 60 to be withdrawn from the sea floor and recovered to the surface for installation at a different location.

The chain 26′ includes a first end 26′A which extends to a floating platform and a second end 26′B which extends to a buoy 61. A chain is connected between the anchor handling vessel and the end 26′B of the chain 26′ that is secured to the buoy 61. A tensioning mechanism on the anchor handling vessel draws the chain 26′ through the device 62. The chain stopper feature of the device 62 prevents the chain from passing through the device 62 in the opposite direction. A line 68 extends to a release mechanism comprising the device 62 whereby the chain may be permitted to pass from the anchor handling vessel toward the floating platform if necessary.

Although preferred embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions of parts and elements without departing from the spirit of the invention. 

1. A method of mooring floating structures comprising the steps of: providing a subsea anchor; providing a chain receiving device; securing the chain receiving device to the anchor; providing a forerunner chain; extending the forerunner chain through the chain receiving device on the anchor; securing the anchor having the chain receiving device secured thereto and having the forerunner chain extending through the chain receiving device in operative engagement with the sea floor; providing a floating structure; providing a mooring line; securing the mooring line in engagement with the floating structure; securing the mooring line in engagement with the forerunner chain; providing an anchor handling vessel; providing a tensioning device on the anchor handling vessel; securing the forerunner chain in operative engagement with the tensioning device on the anchor handling vessel; utilizing the tensioning device on the anchor handling vessel to apply mooring tension to the forerunner chain and thereby applying mooring tension to the mooring line secured to the floating structure; and utilizing the chain receiving device on the anchor to maintain mooring tension. 